Abstract: The progress in systematics of the genus Azolla is reviewed in this article. Recent studies at cytological level proposed that the genus should be re-divided into 2 subgenera:Azolla and Tetrasporocarpia. The subgenus Azolla includes section.Azolla and section Rhizosperma, while the subgenus Tetrasporocarpia comprises 1 species: A.nilotica. Based on the evidence from genetic breeding and isoenzymic analysis, A.mexicana,A.microphylla and A.caroliniana are doubted as separate species in section Azolla. For completely solving the problem in systematics of the genus Azolla, molecular biological technique is regarded as an important means. However,since Azolla is a fern_cyanobacteria symbiont,the heterogeneity of its samples have reduced the reliability of the present data for molecular marking.